1. Field of the Invention
This invention relates to a water-soluble photosensitive resin composition useful in the manufacture of black matrices typically used on the phosphor screen of a color CRT. More specifically, the invention relates to a water-soluble photosensitive resin composition that comprises a water-soluble polymer and a light-sensitive component and that exhibits good enough adhesion to glass substrates to enable the formation of thin-film patterns but which yet can be readily stripped at a subsequent stage. The invention also relates to a method of forming a black matrix pattern on the substrate using said water-soluble photosensitive resin composition.
2. Description of the Related Art
The black matrix for use on the phosphor screen of a color CRT as in a color television set has a light absorber (e.g., graphite) formed in a specified pattern on the inner surface of a glass panel, with the non-patterned areas consisting of many small holes (black matrix holes) or stripes (black stripes). The black matrix holes or stripes are filled with the phosphors of three primary colors, typically red(R), green(G) and blue(B), to provide a pattern of three primary phosphor dots. Electron beams emitted from electron gun(s) are passed through the perforations in a shadow mask to selectively stimulate the phosphor dots, thereby forming a color image on the panel.
A color CRT of this shadow-mask type requires that the positions of the phosphor dots formed on the inner surface of the glass panel should be in registry with those of the perforations in the shadow mask (through which electron beams will pass); if this requirement is not met, the emitted electron beams will either fail to strike the intended phosphors or strike unwanted phosphors, thereby reducing the fidelity of color reproduction. Hence, it is necessary in the formation of a black matrix pattern to provide the inner surface of the glass panel with black matrix holes or stripes in positions that are in good registry with the perforations in the shadow mask. Under the circumstances, the shadow mask for use on a color CRT is commonly employed as an exposure mask in the formation of black matrix patterns.
A customary process of forming a black matrix pattern comprises the steps of applying a water-soluble photosensitive resin composition onto the inner surface of a panel to form a photoresist layer, exposing the photoresist layer to light through a shadow mask, thereby causing the exposed areas to cure, developing the resist layer to remove the unexposed areas, thereby forming a photocured pattern, thereafter applying a light absorber to the entire surface of the substrate including the photocured pattern and the uncovered areas, and drying the applied absorber to form a black film. Subsequently, the photocured pattern and the overlying black film as well are stripped away to form a black matrix pattern having matrix holes or stripes formed in positions that are in registry with the perforations in the shadow mask. The thus formed matrix holes or stripes are filled with the phosphors of three primary colors, R, G and B, to form a phosphor screen having patterns of the three primary phosphors.
The water-soluble photosensitive resin compositions conventionally used in the formation of black matrix patterns are based either on a system consisting of polyvinylalcohol and bichromates or on a system consisting of water-soluble polymers and water-soluble bisazide compounds. However, these systems have their own problems. The first system consisting of polyvinylalcohol and bichromates has the disadvantage of experiencing increasing changes in sensitivity with time, having low resolution and causing an increase in the crosslinked region due to dark reaction that occurs after the termination of exposure. Therefore, if exposure, development and phosphor filling steps are repeated through three cycles in order to form successive patterns of three primary phosphors, the light being applied to the photoresist layer will inevitably overlap between patterns. In such case there is high likelihood for the occurrence of a phenomenon generally called "docking" in which any two adjacent patterns of different colored phosphors that should ideally be separated from each other come into contact. This has been a problem of particularly great concern in "gap exposure" which is performed with the mask spaced from the photoresist layer.
With a view to getting around the "docking" difficulty, several proposals have been made as exemplified by the teaching of Unexamined Published Japanese Patent Application(kokai) No.79970/1973, which relates the method of using polyvinylpyrrolidone as the water-soluble polymer component of a water-soluble photosensitive resin composition, the other component of which is a water-soluble bisazide compound. According to the teaching of that patent, the use of polyvinylpyrrolidone experiences reciprocity law failure, i.e., an effect in which the crosslinking caused by light illumination will hardly proceed if the integrated illumination is below a certain level; hence, the degree of crosslinking of the photoresist layer has such a profile that it increases abruptly in areas relatively close to the center of the beam passing hole whereas it drops markedly farther away from the center. Hence, the degree of crosslinking near the peripheral edge of the beam passing hole is short of the minimum level necessary to form dots and the diameter of the resulting dots is sufficiently smaller than that of the beam passing hole to ensure against "docking". Systems consisting of water-soluble polymers and water-soluble bisazide compounds as taught in Unexamined Published Japanese Patent Application(kokai)No.79970/1973, supra, have high resolution but, on the other hand, they have low sensitivity and adhere so poorly to glass substrates that contact enhancing agents such as water-soluble silane coupling agents have to be added in amounts greater than certain levels; in addition, the thickness of the photoresist layer cannot be substantially reduced because pattern washout will occur during development unless the photoresist layer is formed in thicknesses greater than certain levels. Besides these manufacturing problems from the viewpoints of efficiency and cost, the photoresist layer cannot be completely stripped away and phosphors will adhere to the residual photoresist layer in the subsequent step, which can be a cause of color mixing.
A system that has a light-sensitive group introduced into the water-soluble polymer has been proposed as one that is improved in pattern configuration and resolution but this approach has not yet been commercialized due to difficulties in sensitivity and stability.
The present inventor conducted intensive studies in order to solve the aforementioned problems of the prior art and found that they could be solved by a water-soluble photosensitive resin composition based on a system consisting of a water-soluble polymer and a light-sensitive component, in which either polyvinylpyrrolidone or a copolymer of vinylpyrrolidone and vinylimidazole or both are used as said water-soluble polymer and in which a high-polymer compound having a specified structural unit is used as the light-sensitive component. The present invention has been accomplished on the basis of this finding.